Multi-carrier modulation (MCM) digital communication schemes, such as orthogonal frequency division modulation (OFDM) for wireless communication and discrete multitone (DMT) for digital subscriber lines (DSL) are being rapidly adopted in an effort to efficiently deliver high-speed data services. Such schemes make use of recent advances in digital signal processing (DSP) technology to replace many of the functions previously performed in analog circuitry, with more efficient and robust digital implementations.
MCM is able to adequately overcome the effects of severe frequency-dependent attenuation and dispersion present on many communications links. It does this by dividing the physical channel in frequency into a large number of equally spaced subchannels. The variable attenuation present in wideband channels can be overcome by measuring the magnitude response of the subchannels using standardised test signals. If the channel is fading slowly enough, e.g. in a typical DSL channel or in a wireless channel between two slowly moving or static transceivers, the modems at each end of the line can then allocate bits adaptively to the subchannels according to the received signal to noise ratio (SNR) in order to find a compromise between bit-error rate (BER) and transmission rate. If the channel is fading more quickly then bits may be evenly allocated across subchannels. By measuring the response in each subchannel, the effects of dispersion can be overcome by applying a simple phase correction. This frequency-domain representation of the signal used for coding and decoding makes use of the fast Fourier transform (FFT) and its inverse (IFFT). Integrated circuits for performing the IFFT and FFT at sufficiently high enough rates for wideband communication have only become readily available relatively recently. The sampling rate is chosen so that frequency symmetric pairs of bins in the FFT represent the subchannels.
The existing MCM techniques commonly apply quadrature-amplitude modulation (QAM) in each subchannel. The transmitter packs data bits in blocks into FFT bins (subchannels) using QAM constellations of various sizes, according to the measured capacity, performs the IFFT, and recovers the bits from the QAM constellations in each pair of bins. A predetermined delay is inserted between each symbol to prevent intersymbol interference (ISI) caused by the channel response. This space is usually filled by prefixing the symbol with the last few samples of that symbol. This simplifies the process of synchronization in the receiver.
A problem that is encountered when implementing MCM schemes of the types described is that the time domain signals which result after the IFFT operation may exceed the linear range of the transmit amplifier so that clipping occurs. A related problem is that many MCM systems typically exhibit a large peak-to-average power ratio (PAR). Signals that exhibit a high PAR are problematical in practical systems because the digital-to-analog (DAC) and analog-to-digital converters (ADC) have only a finite range of voltages over which they can transmit and receive and only a finite resolution. If the PAR is too high then either the signalling waveforms will be frequently clipped or there will be inadequate resolution when synthesized in the DAC or sampled in the ADC. It would be desirable if an embodiment of the present invention addressed the problem of clipping discussed above.
The computational overhead involved in implementing MCM of the prior-art type described is dominated by implementation of FFT/IFFT algorithms to decode and code symbols. As is well known, these algorithms require O(N log N) arithmetic computations on a symbol of N samples. It is an object of the present invention to provide a method and apparatus for MCM that is of lower computational complexity than the prior-art FFT/IFFT based schemes and preferably while maintaining a low PAR.